1. Field of the Invention
This invention relates generally to microfabricated devices, and more particularly to three-dimensional devices fabricated with high vertical to horizontal aspect ratios. This invention results in high-aspect-ratio devices that may be fabricated with integrated circuitry on the same substrate using conventional microfabrication techniques.
2. Description of the Related Art
MicroElectroMechanical Systems (MEMS) combine mechanical structures and microelectronic circuits to create devices. MEMS have many useful applications such as microsensors and microactuators. Examples of microsensors include inertial instruments such as accelerometers and gyroscopes, detectors for gasses such as carbon-monoxide, and pressure sensors. Examples of microactuators include optical mirrors used for video displays and disk-drive head actuators used for increasing track density. Passive mechanical structures may also be fabricated in a MEMS technology such as a gimbal for a disk-drive head actuator.
Many MEMS-based devices utilize electrical circuits combined with air-gap capacitors to sense motion, or to apply electrostatic forces to a movable structure. Air-gap capacitors are often formed between sets of capacitor plates anchored to a substrate interleaved with plates attached to a movable structure. The performance of many capacitive-based MEMS improves as: 1) the overlap area of capacitor plates increases, 2) the distance between the stationary and movable capacitor plates decreases, 3) the compliance of the structure varies dramatically in different directions, 4) the mass of the structure increases, and 5) the ratio of parasitic capacitance to active sense capacitance decreases. Each of these performance issues is enhanced using high-aspect-ratio semiconductor technologies, wherein thickness or depth of fabricated structures is much larger than small lateral dimensions such as width of flexible beams and gaps between capacitor plates.
Electrical interfaces for capacitive-based MEMS require electrically isolated nodes spanned by one or more variable capacitors such as an air-gap capacitor. Thus, capacitive interfaces using capacitors formed between structural elements require electrical isolation between these structural elements when the structural elements are formed out of a single conductive substrate.
The performance of devices such as accelerometers and gyroscopes may benefit from combining high-aspect-ratio structures with circuits integrated in the same substrate. Hence, a high-aspect-ratio structure etched into a single-crystal silicon substrate that also contains integrated circuits is of particular interest. Of even greater interest is a process sequence that yields structures and circuits in the same substrate but does not significantly alter complex and expensive circuit fabrication processes. Such a process sequence enables cost-effective manufacture of devices comprising integrated circuits and structures on a single substrate.
For improved performance, an integrated MEMS process should address three issues. First, the structural elements should be formed by a high-aspect-ratio process. Second, fabrication of mechanical structures should have a minimal impact on fabrication of associated circuitry on the same substrate. Third, structural elements should be electrically isolated from one another, the substrate, and circuit elements except where interconnection is desired.
The invention, roughly described, comprises a microelectromechanical structure and method of forming the structure. In one aspect, the method includes: providing a handle substrate; providing a device substrate in which high-aspect-ratio structures and optional integrated circuitry will be fabricated; forming one or more filled isolation trenches within a recessed cavity on a first surface of the device substrate or alternatively forming one or more filled isolation trenches on a first surface of the device substrate and forming a recessed cavity on a first surface of the handle substrate; bonding the first surface of the device substrate to the handle substrate; removing a substantially uniform amount of material from the second surface of the device substrate to expose at least one isolation trench; optionally forming circuits and interconnection on a second surface of the device substrate; and etching a set of features in the second surface of the device substrate to complete the definition of electrically isolated structural elements. The structural-definition etch may also provide electrical isolation among various structural elements, and provide electrical isolation between structural elements and the device substrate except at mechanical attachment points between structural elements or structural elements and the device substrate. The isolation trenches and an insulating material deposited in the isolation trenches may provide electrical isolation at these mechanical attachment points. Structural elements may include, but are not limited to one or more of the following: beams, flexures, springs, levers, proof-masses, fixed air-gap capacitors, and variable air-gap capacitors.
In a further aspect of the invention, a device includes: a device substrate having a first surface, a second surface, and a semiconductor layer; a handle substrate, the first surface of the device substrate bonded to the handle substrate; one or more first trenches formed in the device substrate, the first trenches extending from the second surface of the device substrate through the device substrate towards the handle substrate; a dielectric within the first trenches; one or more cavities disposed below the second surface of the device layer, a cavity enclosing a portion of at least one trench; at least one second trench formed in the second surface of the device substrate, the second trench completing definition of one or more micromechanical devices. The second trench may provide electrical isolation among various structural elements, and provide electrical isolation between structural elements and the device substrate except at mechanical attachment points between structural elements or structural elements and the device substrate. The first trenches provide electrical isolation at these mechanical attachment points. Micromechanical elements may include, but are not limited to one or more of the following: beams, flexures, springs, levers, proof-masses, fixed air-gap capacitors, and variable air-gap capacitors.
The invention allows conducting material to be deposited over exposed isolating trenches to provide electrical connectivity to the structural elements. Conducting material deposited over exposed isolating trenches may also be used to interconnect among electrically isolated circuit and structural elements. Circuit elements may be formed in the device substrate including active devices such as transistors. A single insulating material deposited or grown in the trenches may entirely fill the trenches or an additional filler material may be added to complete filling of trenches after an initial deposition of a first insulating material. The handle or device substrate may include single-crystal or epitaxial silicon, the isolation material may include silicon dioxide or silicon nitride, and the trench fill material may include polycrystalline silicon.
Advantages of the invention include improving the performance of microstructures fabricated in accordance with the invention due to the high-aspect-ratio fabrication that yields larger mass, air-gap capacitors with larger sensitivity, and improved suppression of undesired deflections particularly deflections out of the plane of the device substrate. Fabrication of devices in accordance with the invention may provide mechanical structures mechanically free from the handle substrate without a wet release step. Fabrication of devices in accordance with the invention may also provide a means for alignment of photolithographic masks to a nominally hidden recessed cavity, enabling accurate placement of mechanical structures and optional interconnect or circuits with respect to the recessed cavity with standard photolithographic tools. Fabrication of devices in accordance with the invention may provide substantially voidless isolation trench fill while maintaining good electrical isolation across isolation trenches. Fabrication of devices in accordance with the invention may furthermore provide lower parasitic capacitances in proportion to active sense capacitance than prior art inventions since the invention allows for the use of lower dielectric constant isolation materials than prior art solutions. When circuits are co-fabricated on the device substrate with the mechanical device, performance of devices fabricated in accordance with the invention may be improved due to the proximity of interface circuitry built into the same substrate as the microstructure. Furthermore, when circuits are co-fabricated on the device substrate with the mechanical device, the handle substrate is comprised of heavily doped silicon, the device substrate is comprised of silicon, and the two substrates are bonded by either fusion or thermal compression bonding, a low resistance contact to the back-side of the circuits may be attained.